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A few PowerPoint or Keynote slides are used to set the historical context of the project and to introduce the general problem (see "Presentation Files" and "Instructors Notes" below). Bouncing a rubber ball in class from differing heights above the floor, and letting students see and hear the effects of differing travel times, helps students understand that longer travel time in a reflection experiment indicates a deeper reflector. The relevant parts of YouTube videos are shown (see links under "Other Materials" below). Have the students measure the time interval between the explosion and the impact a few times while showing the videos. One of the correspondents talks about (and attempts to show) how the bridge vibrated after the explosion. Pass out the worksheet and the raw seismograms related to the Hoan demolition experiment. Then, with a copy of the seismograms projected onto the screen, hold an initial discussion of how to interpret the graphics: note the time scale, discuss what the different wave amplitudes mean, and so on. Then cluster into groups of 2-4 students and have each group try to "pick" the first arrivals of  the explosion-induced direct wave,  the impact-induced direct wave, and  the corresponding reflected waves. Depending on the type of students involved (intro non-geologists, intro geology/geophysics, geophysics), the teacher can provide more or less assistance in picking the arrivals of the direct and reflected waves. Work through the quantitative material on the worksheet. Questions about how to handle uncertainty always occur, and if the students do not admit to having questions about this the teacher should ask them how they handle uncertainties. In a nutshell, the resultant uncertainty associated with the sum or difference in two numbers are the sum of the two uncertainties. For example, (23 Â 2) + (14 Â 1) = 37 Â 3. The resultant uncertainty associated with the product of two numbers can be estimated with the sum of the fractional (or percentage) uncertainties. For example, the percentage uncertainty of 23 Â 2 is (2/23) or 8.7% and the percentage uncertainty of 14 Â 1 is (1/14) or 7.1 %, so (23 Â 2) x (14 Â 1) = 322 Â 51 because (8.7% + 7.1%) = 15.8% and 15.8% of 322 is ~51. For a nice summary of simple uncertainty calculations, refer to http://spiff.rit.edu/classes/phys273/uncert/uncert.html or http://webpages.ursinus.edu/lriley/ref/unc/unc.html, or the statistics resources on the SERC website. When the worksheets are completed, recap the experiment and compare the results with a map of crustal thicknesses for North America (e.g., Braile, 1989, Fig. 23B). Finally, it is nice to have the students evaluate the experience as homework.
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